The present invention relates to a device for the longitudinal drawing of thermoplastic preforms possessing a closed base and an open neck, during the blow moulding of these preforms, for the purpose of producing hollow bodies.
At the present time, blow-moulding techniques are very widely used for the production of thermoplastic hollow bodies.
According to one of the most common techniques, preforms having a closed base and an open neck are produced in a first stage, and then, in a second stage, these preforms are blow-moulded in order to produce the desired hollow bodies. In general, the preforms are produced by injection moulding or blow moulding and have a diameter and a height which are considerably smaller than the diameter and the height of the hollow bodies to be produced. Furthermore, these preforms are advantageously subjected to axial drawing, before or during the final drawing which accompanies blow moulding. This kind of drawing proves particularly beneficial if the preforms have been subjected to prior thermal conditioning suitable for producing molecularly oriented hollow bodies during the final moulding. In fact, in this case, the longitudinal drawing of the preforms produces an axial molecular orientation in the latter, whereas the drawing caused by the final blowing produces a radial molecular orientation. Hollow bodies are thus obtained in which the walls are molecularly bioriented in orthogonal directions, and which are characterized by exceptional mechanical properties.
In general, the longitudinal drawing of the preforms is carried out by means of drawing rods which are introduced into the preforms through their open neck, and which exert a thrust on their closed base. The travel of the drawing rods is generally calculated so that the preforms, held by their neck, are drawn longitudinally until they reach a length which is virtually equal to, or slightly less than, the height of the desired hollow bodies. Since hydraulic or pneumatic drive systems are incapable of displacing the drawing rods with sufficient uniformity, the latter are generally actuated mechanically, for example by means of cams.
However, the use of mechanically actuated drawing rods exhibits a very serious disadvantage. In fact, it frequently happens that, during their necessary displacement, the drawing rods exert too great a thrust on the bases of the preforms and perforate them. An incident of this kind can occur, for example, if, for any reason, a preform is at a temperature which is less, even slightly less, than the temperature envisaged for drawing. In such a case, the preform is not blown when the expansion fluid is introduced and it usually remains attached to the mould or to the blow nozzle when the mould is opened. This consequently necessitates the continual presence of an operator for removing the perforated preforms by hand, in order to free the blow moulds.